Combustion System Using Recycled Flue Gas to Boost Overfire Air

ABSTRACT

The present application and the resultant patent provide a combustion system. The combustion system may include a combustion chamber for combusting a flow of fuel and a flow of air to a flow of flue gases, an overfire air system in communication with the combustion chamber, and a flue gas return line in communication with the overfire air system such that a recycled flue gas flow mixes with an overtire air flow before entry into the combustion chamber.

TECHNICAL FIELD

The present application and the resultant patent relate generally to combustion systems and more particularly relates to combustion systems using a recycled flue gas flow to boost an overfire air flow for a reduction in emissions and an increase in efficiency.

BACKGROUND OF THE INVENTION

Combustion systems are used in numerous industrial environments to generate heat and hot gases. For example, boilers and furnaces typically burn hydrocarbon fuels such as oil and coal in stationary combustors to produce heat to raise the temperature of a fluid such as water. Industrial combustion systems typically employ various burner elements to combust the fuel and air injectors to provide combustion air to ensure complete combustion.

An industrial furnace or a utility furnace such as a boiler, whether gas or fossil fired, typically includes a lower combustion zone and a generally vertically extending flue gas passage. The air introduced into such a combustion system may be staged, i.e., the air may be introduced into the system in multiple stages to optimize combustion and minimize emissions. In air staging, the primary air is mixed with the fuel as both are injected into a combustion zone. Secondary air (air without fuel) may be injected in the primary combustion zone and also may be injected downstream of the primary combustion zone as in the case of overtire air.

Overfire air typically is injected into the flue gas at a location downstream of the primary combustion zone. Overtire air staging reduces the flow of combustion air provided to the primary combustion zone so as to suppress the formation of emissions such as nitrogen oxides. The reduced oxygen in the primary combustion zone increases the level of unburned hydrocarbons in the flue gas. The overtire air then completes combustion of the unhurried hydrocarbons, which are converted into carbon dioxide and water.

Although known overtire air systems generally are effective in reducing undesired emissions, such emissions control may come at an efficiency cost with respect to the overall combustion system. There is thus a desire for improved combustion systems using overtire air so as to provide adequate or increased emissions control with little or no impact on overall system efficiency.

SUMMARY OF THE INVENTION

The present application and the resultant patent thus provide a combustion system. The combustion system may include a combustion chamber for combusting a flow of fuel and a flow of air to a flow of flue gases, an overfire air system in communication with the combustion chamber, and a flue gas return line in communication with the overfire air system such that a recycled flue gas flow mixes with an overfire air flow before entry into the combustion chamber.

The present application and the resultant patent further provide a method of operating a combustion system. The method may include the steps of combusting a flow of air and a flow of fuel into a flue gas flow in a combustion zone of a combustion chamber, diverting a portion of the flow of air as an overtire air flow, recycling a portion of the flue gas flow, mixing the overfire air flow and the recycled portion of the flue gas flow, and injecting the overfire air flow and the recycled portion of the flue gas flow in the combustion chamber downstream of the combustion zone.

The present application and the resultant patent further provide a combustion system. The combustion system may include a combustion chamber for combusting a flow of fuel and a flow of combustion air to a flow of flue gases in a combustion zone, an overfire air system in communication with the combustion chamber downstream of the combustion zone for diverting a portion of the flow of combustion air as an overfire air flow, and a flue gas return line in communication with the overfire air system such that a recycled flue gas flow mixes with the overfire air flow before entry into the combustion chamber.

These and other features and improvements of the present application and the resultant patent will become apparent to one of ordinary skill in the art upon review of the following detailed description when taken in conjunction with the several drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic view of a combustion system as may be described herein using an overfire air system.

DETAILED DESCRIPTION

Referring now to the drawing, in which like numerals refer to like elements through out, FIG. 1 shows a combustion system as may be described herein. The combustion system 100 may be a fossil fuel fired boiler, furnace, and the like. The combustion system 100 combusts a flow of fuel 110 with one or more flows of air 120. The fuel 110 may be a fossil fuel such as a supply of coal 130 and the like. The supply of coal 130 may be stored in a coal bunker 140 or other type of storage. The coal 130 may be pulverized as needed in a mill 150 or a similar type of device. The flow of air 120 may include a primary combustion air flow 160. The primary combustion air flow 160 mixes with the pulverized coal 130 in the mill 150 for transport therewith. The one or more air flows 120 may have one or more fans 170 or other type of air movement devices associated therewith.

The combustion system 100 may include a combustion chamber 180. The combustion chamber 180 may have a somewhat elongated shape although any desired size, shape, or configuration may be used herein. As will be described in more detail below, the combustion chamber 180 may include a combustion zone 190 for combusting the flow of fuel 110 and the flow of air 120, a reburn zone 200 downstream thereof, and a burnout zone 210 further downstream. Other components and other configurations may be used herein.

The combustion chamber 180 may include one or more main burners 220 or other types of combustion devices. Although a coal fired main burner 220 is described herein, gas fired, oil fired, or other types of burners and the like also may be used herein. The main burner 220 includes a fuel input 230. The fuel input 230 may be in communication with the pulverized coal 130 and the primary combustion air flow 160 from the mill 150. The main burner 220 also may have an air input 240. The air input 240 may be in communication with a combustion air flow 250 of the one or more air flows 120. The one or more air flows 120 also may include a burner core air flow 260, a furnace leakage flow 270, and the like. The flow of fuel 110 and the flow of air 120 thus are ignited within the combustion chamber 180 by the main burner 220 to form a combustion flue gas flow 280. The combustion flue gas 280 flows downstream towards the reburn zone 200.

The combustion system 100 also includes an overfire air system 290. The overtire air system 290 may include one or more overfire air injector 300. The overfire air injectors 300 may be in communication with a portion of the combustion airflow 250. The overfire air system 290 thus divers the portion of the combustion air flow 250 from the main burner 220 for combustion downstream of the combustion zone 190 as an overfire air flow 305.

The overfire air system 290 also may include a flue gas return line 310 to provide a recycled flue gas flow 315. The flue gas return line 310 may be in communication with the overtire air injector 300 and the combustion air flow 250. The flue gas return line 310 also may be in communication with one or more flue gas heat exchangers 320 before reaching the overtire air injector 300. The flue gas heat exchangers 320 may be in communication with the one or more incoming flows of air 120. The flue gas heat exchangers 320 thus may cool the recycled flue gas flow 315 while heating the incoming flows of air 120. The portion of the flue gas flow 280 not diverted to the overtire air system 290 as the recycled flue gas flow 315 may pass through an electrostatic precipitator 330 or a bag house and may exit the combustion system 100 via a stack 340 or other type of exhaust. Alternatively, the remainder of the flue gas 280 flow also may be used for other purposes in the overall combustion system 100 or otherwise.

In use, about ten percent (10%) of the overall flue gas flow 280 may be used as the recycled flue gas flow 315 and forwarded via the flue gas return line 310 to the overtire air injectors 300 of the overtire air system 290. Other percentages and other types of flows also may be used herein. The recycled flue gas flow 315 thus mixes with the overfire air flow 305 diverted from the combustion air flow 250. This mixture of the recycled flue gas flow 315 and the overfire air flow 305 may be injected into the combustion zone 190 at a speed higher than about sixty (60) meters per second or so. Other speeds may be used herein. The high speed of injection promotes good mixing of the overfire air flow 305 and the flue gas flow 280. Good mixing thus may reduce effectively combustible emissions therein.

The use of the recycled flue gas flow 315 thus boosts the flow of the overfire air flow 305 to a higher mass, velocity, and momentum. Such an increase in momentum may control unburned hydrocarbons and carbon monoxide. Less overtire air 305 may be required for maintaining the combustible emissions level at a base line. Specifically, with a reduction of the overtire air flow rate, the oxygen level within the combustion zone 190 may be reduced so as to improve the efficiency of the overall combustion system 100.

The combustion system 100 thus removes a portion of the combustion air flow 250 from the combustion zone 190 and injects the portion as the overfire air flow 305 at an upper elevation so as to complete combustion. Because the combustion air flow 250 is reduced, emissions of nitrogen oxides and the like may be lowered. Injecting the overfire air flow 305 at a higher velocity and momentum thus helps to reduce combustible emissions without impacting on boiler or overall system efficiency. Specifically, the increase velocity and momentum of the overtire air flow 305 may control effectively combustibles such that less air flow is needed. By reducing the total air flow, the overall efficiency of the combustion system 100 may be improved. Using the recycled flue gas flow 315 thus further reduced emissions while maintaining or improving overall system efficiency.

The overfire air system 290 with flue gas recycling may be used to modify existing combustion systems 100 given that modified wall openings may not be required. Moreover, such a modification may be relatively inexpensive to install and may require little downtime.

It should be apparent that the foregoing relates only to certain embodiments of the present application and the resultant patent. Numerous changes and modifications may be made herein by one of ordinary skill in the art without departing from the general spirit and scope of the invention as defined by the following claims and the equivalents thereof. 

1. A combustion system, comprising: a combustion chamber for combusting a flow of fuel and a flow of air to a flow of flue gases; an overfire air system in communication with the combustion chamber; and a flue gas return line in communication with the overtire air system such that a recycled flue gas flow mixes with an overfire air flow before entry into the combustion chamber.
 2. The combustion system of claim 1, wherein the flow of fuel comprises a supply of coal.
 3. The combustion system of claim 2, further comprising a mill in communication with the supply of coal.
 4. The combustion system of claim 1, further comprising one or more main burners positioned about the combustion chamber and in communication with the flow of fuel and the flow of air.
 5. The combustion system of claim 4, wherein the combustion chamber comprises a combustion zone about the one or more main burners.
 6. The combustion system of claim 5, wherein the overtire system is positioned downstream of the combustion zone.
 7. The combustion system of claim 1, wherein the overtire system comprises one or more overfire air injectors.
 8. The combustion system of claim 1, wherein the flow of air comprises a flow of combustion air and wherein the overtire air flow comprises a portion of the flow of combustion air.
 9. The combustion system of claim 8, further comprising one or more flue gas heat exchangers in communication with the flue gas return line and the flow of combustion air.
 10. The combustion system of claim 1, wherein the recycled flue gas flow comprises about ten percent (10%) of the flue gas flow.
 11. The combustion system of claim 1, wherein the recycled flue gas flow and the overfire air flow comprise an entry speed into the combustion chamber of about sixty (60) meters per second or more.
 12. A method of operating a combustion system, comprising: combusting a flow of air and a flow of fuel into a flue gas flow in a combustion zone of a combustion chamber; diverting a portion of the flow of air as an overtire air flow; recycling a portion of the flue gas flow; mixing the overfire air flow and the recycled portion of the flue gas flow; and injecting the overfire air flow and the recycled portion of the flue gas flow in the combustion chamber downstream of the combustion zone.
 13. The method of claim 12, further comprising the step of exchanging heat between the flow of air and the recycled portion of the flue gas flow upstream of the combustion chamber.
 14. The method of claim 12, wherein the step of recycling a portion of the flue gas flow comprises recycling about ten percent (10%) of the flue gas flow.
 15. The method of claim 12, wherein the step of injecting the overtire air flow and the recycled portion of the flue gas flow comprises injecting the overtire air flow and the recycled portion of the flue gas flow at about sixty (60) meters per second or more.
 16. A combustion system, comprising: a combustion chamber for combusting a flow of fuel and a flow of combustion air to a flow of flue gases in a combustion zone; an overfire air system in communication with the combustion chamber downstream of the combustion zone for diverting a portion of the flow of combustion air as an overfire air flow; and a flue gas return line in communication with the overtire air system such that a recycled flue gas flow mixes with the overtire air flow before entry into the combustion chamber.
 17. The combustion system of claim 16, further comprising one or more main burners positioned about the combustion zone and in communication with the flow of fuel and the flow of combustion air.
 18. The combustion system of claim 16, wherein the overtire system comprises one or more overfire air injectors.
 19. The combustion system of claim 16, wherein the flow of fuel comprises a supply of coal.
 20. The combustion system of claim 16, further comprising one or more flue gas heat exchangers in communication with the flue gas return line and the flow of combustion air. 